Robust, versatile, and high-throughput biosensors are required for genomic and proteomic research applications, for high throughput screening in drug development, for rapid point of care assays and, potentially for in vivo imaging applications. In particular, there is a need for assay methodologies that a) are highly sensitive, b) minimize sample preparation and allow measurements in turbid/obscure media, c) measure widely different types of molecules using the same format and instrument and, d) can be run in single tube point of care or high throughput screening formats. We have developed a novel magnetic nanosensor technology that uses "magnetic relaxation switches" (MRSW), i.e. magnetic nanoparticles that are induced to switch between dispersed and clustered states by analytes, with changes in the spin-spin relaxation time (T2) of adjacent water molecules. The broad goals of this application are to optimize and extend MRSW methodology, and to develop MRSW assays for key areas of biomedical research and needs. Specific aim 1 involves the optimization of nanoparticles for MRSW assays, and improvements and extensions of the MR based instrumentation the method uses. In Specific Aim 2 we will apply the MRSW methodology to key issues of cancer biology (telomerase and telomere length), microbiology (virus detection and characterization), and panomics.